1. Field of the Invention
The invention relates to a method and a device for determining an unabbreviated channel-impulse response in an OFDM transmission system.
2. Discussion of the Background
Cellular common-frequency networks (SFN single-frequency networks) are increasingly used in digital radio technology in order to reduce the required transmission power. In a cellular single-frequency network, an identical transmitted signal is transmitted in a time-synchronous and frequency-synchronous manner by several transmitters stationed in regionally-distributed cells. In a cellular single-frequency network, the corresponding received signal is composed from the additive superpositions of the individual transmitted signals delayed by the distance-dependent signal-delay time and convoluted with the independent channel-impulse response.
In view of the identical transmitted signals, the transmission channel in a cellular single-frequency network can therefore be characterized by a virtual channel-impulse response. The length of this virtual channel-impulse response depends upon the maximum delay-time difference between two transmitters disposed at the maximum distance from one another and the maximum alternate-routing-delay of the multi-route channel. In view of the cell size of 30 kilometers or more, which is conventional in digital radio technology, channel-impulse-response lengths of more than 100 μs can therefore occur. The virtual channel-impulse response in this context is determined by a few echo-clusters with a length of typically less than 10 μs displaced relative to one another in time, which are generally allocated in each case to one transmitter.
An accurate knowledge of the behaviour of the transmission channel is essential for the correct operation of a single-frequency network. The measurement of the transmission channel by estimation required for this purpose determines an optimum reconstruction of the virtual channel-impulse response.
In order to determine the virtual channel-impulse response, in an OFDM (Orthogonal Frequency Division Multiplex) transmission system, pilot symbols are transmitted in a given frequency-time raster of the frequency-time transmission frame composed of frequency carriers and symbol durations. Within one symbol duration of the frequency-time transmission frame, the pilot symbols to be transmitted for the determination of the channel-impulse response are arranged in a guard interval disposed before or after the useful-data interval.
So long as the duration of the virtual channel-impulse response is shorter than the length of the guard interval, the virtual channel-impulse response can be reconstructed within the guard interval without interference. The reconstruction of the useful-data symbol transmitted in the useful-data interval is also implemented without difficulty.
If the virtual channel-impulse response extends over the length of the guard interval, inter-symbol interference occurs between the pilot symbols and the useful-data symbols, which impairs the reception of the useful-data symbols and also no longer allows the reconstruction of the channel-impulse response from the transmitted pilot symbols.
According to the prior art—as disclosed, for example, in DE 100 05 287 A1—the channel estimation for the correct reconstruction of the useful-data symbols is implemented on the basis of pilot symbols as follows:
The pilot symbols are positioned according to FIG. 1A in a raster of respectively (NF) frequency carriers within the transmission bandwidth consisting of a total of (NC) frequency carriers. The discrete channel-transmission function (Ĥk) of the OFDM transmission channel determined in the receiver from the pilot symbols by means of an estimator provides an impulse response (h(n)) continued periodically in the time domain with the periodicity
      (                  N        C                    N        F              )    ,as shown in FIG. 1B. The estimation of the channel-transmission function ({tilde over (H)}k) in the frequency carriers not occupied by pilot symbols is implemented according to FIG. 1C by interpolation from the estimated values of the channel-transmission function (Ĥk) in the frequency carriers defined with pilot symbols by means of interpolation filters. The interpolation determines a windowing or respectively band limitation of the impulse response (h(n)) in the time domain as shown in FIG. 1D at the level of the filter length (FI) of the interpolation filter (grey-dotted, trapeze-shaped region in FIG. 1D).
As a prerequisite for a correct reconstruction of the channel-impulse response, the duration of the channel-impulse response must be kept smaller than the period of the periodically-continued individual channel-impulse responses by appropriate dimensioning of the frequency raster (NF) of the pilot symbols in order to prevent an aliasing between the individual periodically-continued channel-impulse responses. For this purpose, the ratio of the frequency raster (NF) of the pilot symbols relative to the system-bandwidth (NC) of the OFDM transmission channel must be designed to be larger than the length (NG) of the guard interval as shown in equation (1).
                              N                      G            ⁢                                                                ≤                              N            C                                N                          F              ⁢                                                                                                      (        1        )            
Moreover, the filter length (FI) of the interpolation filter should be designed in such a manner that the periodic continuations of the channel-impulse response are suppressed. The filter length (FI) of the interpolation filter must therefore be designed to be smaller than the ratio of the frequency raster (NF) of the pilot symbol relative to the system bandwidth (NC) of the OFDM transmission channel.
If the channel-impulse response provides a relatively-shorter duration than the filter length FI of the interpolation filter, the channel-impulse response can be correctly reconstructed according to FIG. 1D.
However, if, as illustrated in FIG. 1E, the duration of the channel-impulse response is longer than the selected filter length FI of the interpolation filter, part of the channel-impulse response is suppressed by the band limitation of the interpolation. In this case, a correct reconstruction of the channel-impulse response is not possible.
In order to remove the negative influence of the band limitation of the interpolation filter on the reconstruction of the channel-impulse response, the interpolation is conventionally replaced by a more-complex channel estimation by means of pilot symbols in a relatively-fine frequency raster. With an estimation of the channel-transmission frequency in the frequency domain by means of pilot symbols relatively-finer by the factor Q (factor Q<1) according to FIG. 2A, the period of the periodically-continued channel-impulse response is increased in the time domain by the same factor Q as shown in FIG. 2B. Consequently, the duration of the channel-impulse response can be prolonged by this factor Q, without causing a faulty reconstruction. If the duration NG of the guard interval in FIG. 2A has not changed relative to the situation in FIG. 1B or respectively 1D and 1E, the channel-impulse response ĥ according to equation (2) can be reconstructed in an error-free manner through an inverse Fourier transform from the estimated channel-transmission function Ĥ, even if the duration of the channel-impulse response ĥ extends beyond the length NG of the guard interval.ĥ=F−1{Ĥ}  (2)
However, an error-free reconstruction of a channel-impulse response, the duration of which extends over the entire symbol duration, presupposes a definition of each of the NC frequency carriers with pilot symbols. Since useful-data symbols can no longer be transmitted in this manner, a procedure of this kind cannot be used in practice with a channel-impulse response with a very long duration to be reconstructed.
While a faulty reconstruction of the channel-impulse response is avoidable for the current operation of an OFDM transmission system through an appropriate choice of the frequency raster NF of the pilot symbols and through an appropriate choice of the filter length FI of the interpolation filter, for the testing of mobile-telephone networks, in which there is no possibility of adapting the frequency raster NF of the pilot symbols and the filter length FI of the interpolation filter in current network operation, it must also be possible to reconstruct the channel-impulse response correctly as presented in FIG. 1E.
With measurements during network operation of the cellular single-frequency network, for example, it is possible to check from the correctly-reconstructed virtual channel-impulse response, whether the maximal delay-time difference of the associated transmitted signals determined by the maximal regional distance between two transmitters of the cellular single-frequency network is disposed within the maximum length of the channel-impulse response capable of being processed by the receiver. Moreover, the individual echoes associated with the transmitter can be identified from the correctly-reconstructed virtual channel-impulse response and, the frequency synchronisation of the individual transmitters can be monitored from the Doppler shift of the echoes associated with the individual transmitters.